1. Field of the Disclosure
The present invention relates to a readout circuit for a touch sensor, and more particularly to a readout circuit which can vary a sensing range of a sensing signal according to variation of a characteristic of a touch sensor.
2. Discussion of the Related Art
These days, a touch screen which enables to input information by touching a screen of various display devices is widely applied to a computer system as an information input device. Since the touch screen enables a user to move or select displayed information by simple touch of the screen with a finger or a stylus, everybody can use the touch screen, easily.
The touch screen provides touch information by sensing a touch and a touch position on the screen of the display device, and the computer system analyzes the touch information to carry out an order. As the display device, a flat display device, such as a liquid crystal display device, a plasma display panel, an organic luminescence diode display device, and the like are used, mostly.
Depending on sensing principles, in touch screen technologies, there are a resistance film type, a capacitive type, a photo type, an infrared rays type, an ultrasonic type, and an electro-magnetic type. In general, the touch screen is fabricated as a panel attached to an upper side of the display device for performing a touch input function. However, since the display device having the touch panel attached thereto is required to fabricate a touch panel separate from the display device and attach to the display device, the display device having the touch panel attached thereto is involved in high production cost, increased thickness and weight of an entire system making mobility poor, and design limitations.
In order to solve those problems, recently, an in-cell touch sensor is developing, in which the touch sensor is built in the display device, such as the liquid crystal display device, or an organic luminescence diode display device. As the in-cell touch sensor, a photo-touch sensor which uses a photo-transistor to perceive the touch with light intensity, and a capacitive touch sensor which perceives the touch according to variation of capacitance, are used, mostly.
The photo-touch sensor perceives the touch with a photo-leakage current from the photo-transistor caused by an incident light or a reflected light by a touch body. The capacitive touch sensor perceives the touch with a change of the capacitance taking place when a small amount of charge moves to a touch point when a conductive body, such as a human body and a stylus, touches the touch sensor.
The readout circuit is provided with an amplifier for comparing a signal from the touch sensor to a reference voltage to generate a touch sensing signal which indicates whether the touch is made or not, and an analog to digital converter ADC for converting an analog signal from the amplifier into a digital sensing signal and forwarding the digital sensing signal to a signal processing unit.
In general, referring to FIG. 1, the ADC has an operative range of 1V with reference to the reference voltage Vref. However, the in-cell touch sensor has a problem in that a base level varies according to process variation and a driving voltage when there is no touch as shown in FIG. 1. For an example, the base level varies with variations of resistance and parasitic capacitance caused by a position of the touch sensor, or, as shown in FIG. 1, the base level becomes the higher as the driving voltage of the touch sensor becomes the higher.
However, if the base level varies with the process variation and the driving voltage of the in-cell touch sensor, there has been a problem in that the sensing sensitivity becomes poor by failure of quantization of a overflowed portion taking place when a touch peak value of the touch exceeds a driving range (Vref˜Vref+1V) of the ADC.
The problem of the poor touch sensing sensitivity can also take place at a touch panel attached to the display device by noise from the display device.